Dye-labeled antibodies have been used in analysis such as immunochromatography because of their reactive specificity. Cyanine dyes having reactive functional groups are often used as a dye to label antibodies (Bioconjugate Chemistry, VOL. 4, No. 2, pp105-111, 1993). The functional groups in the cyanine dyes bond to amino or carboxy groups of the antibodies to form covalent bonds. Twenty to fifty molecules of the dye bond to one molecule of an antibody. The dye-labeled antibodies thus prepared are generally easy to recognize with the naked eye. Such dye-labeled antibodies are used in immunochromatography, for example, to detect a trace of some components like human chorionic gonadotropin (hCG), which is present only in urine from pregnant women.
However, the conventional dye-labeled antibodies had a limited capability in detection for some antigens. A low concentration of substances to be detected rendered the detection difficult.